Influenza viruses have been a major cause of human mortality and morbidity throughout recorded history. Influenza A virus infection causes millions of cases of severe illness and as many as 500,000 deaths each year worldwide. Epidemics vary widely in severity but occur at regular intervals and always cause significant mortality and morbidity, most frequently in the elderly population. Although vaccines against matched influenza strains can prevent illness in 60-80% of healthy adults, the rate of protection is much lower in high-risk groups. Furthermore, vaccination does not provide protection against unexpected strains, such as the H5 and H7 avian influenza outbreaks in Hong Kong in 1997 and Europe and Southeast Asia in 2003 and 2004. Current anti-influenza drugs are limited in their capacity to provide protection and therapeutic effect (Cox and Subbarao 1999; Cox and Subbarao 2000).
Influenza A is a segmented RNA virus of negative-polarity. Genome segments are replicated by a complex of 4 proteins: the 3 polymerase polypeptides (PA, PB1 and PB2) and NP (Nucleoprotein). The 5′ and 3′ terminal sequence regions of all 8 genome segments are highly conserved within a genotype (Strauss and Strauss 2002).
Influenza A viruses can be subtyped according to the antigenic and genetic nature of their surface glycoproteins; 15 hemagglutinin (HA) and 9 neuraminidase (NA) subtypes have been identified to date. Viruses bearing all known HA and NA subtypes have been isolated from avian hosts, but only viruses of the H1N1 (1918), H2N2 (1957/58), and H3N2 (1968) subtypes have been associated with widespread epidemics in humans (Strauss and Strauss 2002).
Since 1997, when H5N1 influenza virus was transmitted to humans and killed 6 of 18 infected persons, there have been multiple transmissions of avian influenza viruses to mammals. Either the whole virus is transmitted directly or gene segments from the avian influenza virus are acquired by mammalian strains. Widespread infections of poultry with H5N1 viruses in Asia have caused increasing concern that this subtype may achieve human-to-human spread and establish interspecies transmission. The species which different types of influenza viruses are able to infect are determined by different forms of the virus glycoproteins (HA, NA). This provides a considerable species barrier between birds and humans which is not easily overcome. Pigs, however, provide a “mixing pot”-able to be infected by both types of virus and thereby allowing the passage of avian viruses to humans. When an individual pig cell is co-infected with both avian and human influenza viruses, recombinant forms can emerge that carry an avian HA genotype but readily infect humans. Avian HA can infect pigs, but not humans. In pigs, during genome segment packaging, it is possible to create a virus with several Avian segments and Human HA and/or NA segments (Cox and Subbarao 2000).
Influenza viruses infect humans and animals (e.g., pigs, birds, horses) and may cause acute respiratory disease. There have been numerous attempts to produce vaccines effective against influenza virus. None, however, have been completely successful, particularly on a long-term basis. This may be due, at least in part, to the segmented characteristic of the influenza virus genome, which makes it possible, through re-assortment of the segments, for numerous forms to exist. For example, it has been suggested that there could be an interchange of RNA segments between animal and human influenza viruses, which would result in the introduction of new antigenic subtypes into both populations. Thus, a long-term vaccination approach has failed, due to the emergence of new subtypes (antigenic “shift”). In addition, the surface proteins of the virus, hemagglutinin and neuraminidase, constantly undergo minor antigenic changes (antigenic “drift”). This high degree of variation explains why specific immunity developed against a particular influenza virus does not establish protection against new variants. Hence, alternative antiviral strategies are needed. Although influenza B and C viruses cause less clinical disease than the A types, new antiviral drugs should also be helpful in curbing infections caused by these agents.
Influenza viruses that occur naturally among birds are called avian influenza (bird flu). The birds carry the viruses in their intestines but do not generally get sick from the infection. However, migratory birds can carry the bird flu to infect domestic chickens, ducks and turkeys causing illness and even death. Avian flu does not easily infect humans but when human exposure is more frequent, such as contact with domestic birds, human infections occur. A dangerous bird flu (H5N1) was first identified in terns in South Africa in 1961 and was identified as a potentially deadly form of flu. Outbreaks of H5N1 occurred in eight Asian countries in late 2003 and 2004. At that time more than 100 million birds in these countries either died or were killed in order to control the outbreak. Beginning in June of 2004 new deadly outbreaks of H5N1 were reported in Asia which is currently ongoing. Human infections of H5N1 have been observed in Thailand, Vietnam and Cambodia with a death rate of about 50 percent. These infections have mostly occurred from human contact with infected poultry but a few cases of human-to-human spread of H5N1 have occurred.
Currently, there is no vaccine to protect humans against H5N1 but research efforts are underway. There are four currently approved influenza medications, amantadine, rimantadine, oseltamivir and zanamivir. Unfortunately, the H5N1 virus is resistant to both amantadine and rimantidine. The remaining oseltamivir and zanamivir may show some efficacy to H5N1 but need to be evaluated more extensively.
In view of the severity of the diseases caused by influenza viruses there is an immediate need for new therapies to treat influenza infection. Given the lack of effective prevention or therapies, it is therefore an object of the present invention to provide therapeutic compounds and methods for treating a host infected with an influenza virus.